Sensitizing planographic plates for photo-lithography



3,375,113 SENSITIZING PL GRAPHIC PLATES FOR PHOTO-LITHOGRAPHY Towers Doggett, Westbrook, vMaine, assignor, by mesne assignments, to Scott Paper Company, County, Pa., a corporatio of Pennsylvania No Drawing. Filed'Sept. '21, 1962, Ser. No. 225,380

This invention relates to the manufacture of photolithographic printing plates. It is more particularly concerned With an improved diazo wash solution for'treating the surface .of a planographic printing plate to make it photosensitive. k

On difliculty with sensitizing photo-lithographic printing Plates With a P -sensit ve d ab co poun h b en that the diazo compound aged rapidly and within a few days became water insoluble and ink receptive. Various means have been used to make the diazo compounds more stable, such as sulfonation by means of sulphites as described in U.S. Patent 2,778,735. While the storage stability of plates sensitized by a sulfonate-d diazo compound is several months or more, the plates are not too sensitive to light and require considerable exposure. The ability of a sulfonated diazo compound to attract the oleaginous printing ink may be inadequate even after prolonged exposure to light. -It has usually been necessary to treat a developed plate with an oleophilic agent to improve the aflinity of the exposed diazo compound for the printing ink.

The present invention is based on the finding that photo-sensitive diazo compounds can be adequately stabilized without appreciably impairing their sensitivity by treatment with phosphoric acid. The photo-sensitivity of the diazo compounds treated with phosphoric acid is four or more times that of the sulfonated diazo compounds; yet plates prepared from the phosphonated compounds have been stored for over one and one half years without spoilage.

The amount of phosphoric acid used to stabilize the diazo compounds is relatively high, in the range of 3.5 to 7.5 parts of weight per part of the diazo compound. This ratio is quite important. If lesser amounts are used, the stability imparted is insuflicient and the diazo compound ages poorly. If greater amounts are used, the ink receptivity of the developed surface is inadequate. The reason for this is not known with certainty. It is not clear at present what portion of the diazo compound is complexed, buffered, reacted with or the like to produce the stabilizing effect without adversely effecting sensitivity. Other mineral acids such as sulphuric and nitric are known to be less effective or deleterious at these concentrations.

The diazo compound used in the practice of this invention is *diazotized para-aminodiphenylamine condensed with formaldehyde and converted to the double salt of a divalent heavy metal. An equal molar amount of formaldehyde is preferably used. The diazonium salt is preferably in water solution in a concentration in the range of 0.5 to 2 weight percent. The phosphoric acid is added by simple mixing to obtain the desired sensitizing wash solution.

The wash solution can be applied to the hydrophilic surface of any suitable planographic printing plate, such as a clay-casein surface on a paper web that has been precoated with a barrier coating. Metal foils, plastic coated papers, plastic sheets, and the like can alse be used. The hydrophilic cellulosic type of coating described in US. 2,778,735 on any suitable base as described therein is preferred. The wash solution is applied in any convenient manner, e.g., roll coating, with an air knife, dip- Delaware 3,375,113 Patented Mar, 26, 9 8

ping, spraying, and the like. In the range of 0:05 to 0.30 grams of the diazo compound (dry weight and excluding acid) per square meter of the surface will normally be used.

Example The ph o-sensiti mp u d used in thi ex mple wa d a otizs para-.am nod p s mins o d ns d i h n equa molar amount o f ldehyde an co ve t d o the doub e alt of zi c chl de- A 0 s w ig t p cen wa e solu ion of t is compo d wa m xed h an 8 percent solution of phosphoric acid in varying proportions. The solutions thus obtained were applied'by an air knife coater to a paper lithographic printing plate made as described in the example of UJS. 2,778,735, except that the phosphoric acid and diazo compound mixture was Insed in place vof the light sensitive coating of the patent. The amount of diazo compound applied was 0.15 grams/ per square meter. The plates were aged for four hours at 70 C. This is considered to be equal to about 6 months natural aging at room temperature. The aged plates were tested using a conventional offset lithographic press with Addressograph-Multigraph Company ML 36 ink and A-M Repelex diluted with water (1 part to 7 parts water).

The negative was a halftone lithographic film negative (-150 lines/inch) with a Lithographic Technical Foundation sensitivity guide. The following results were obtained.

Weight Ratio of Phos- Start-Up on phoric Acid (100%) to Press Printing Results Diazo Compound 0. Satisfactory. Unsatisfactory. 1.70 do Do. 2. 55 do Do. 8. 40 do D0. 4. 25 do Fairly satisfactory. 6. 10 do. Satisfactory. 6. 37 ..do Do. 8. 50 Do.

Plates treated with the 6.37 acid/diazo compound solution were stored for twenty months and then tested. The start-up, toning properties and fidelity of these plates were satisfactory.

The plates prepared with the diazo compound treated with 6.37 parts of phosphoric acid were compared to the plates prepared using the same diazo compound but sul- 1 It might be noted that the same type of planographic plates coated with the diazo compound unstabilized in any manner have a shelf life of less than a month.

Having thus described this invention, what is sought to be protected by Letters Patent is succinctly set forth in the following claims.

What is claimed is:

1. A presensitized printing plate comprising a base material having thereon a coating comprising 1) the double metal salt of the condensation product of a diazotized para-amino-diphenylamine and formaldehyde and (2) phosphoric acid.

2. A photo-lithographic printing plate for use in planegraphic printing comprising a base sheet coated with a lithographic coating and sensitized with a diazo compound stabilized with in the range of 3.5 to 7.5 parts by weight of phosphoric acid per part by weight of said diazo compound, said diazo compound being diazotized para-amino-diphenylamine condensed with formaldehyde and converted to the double salt of a divalent heavy metal.

3. A solution of a stabilized diazo compound for treating a planogra-phic printing plate to make it photo-sensitive comprising a water solution of diazotize-d para-aminodiphenylamine condensed with formaldehyde and converted to the double salt of a divalent heavy metal and in the range of 3.5 to 7.5 parts by weight thereon ofphosphoric acid per part by Weight of said diazo compound.

4. A photo-sensitive planographic printing plate comprising a base having a planographic printing surface coated with in the range of 0.05 to 0.30 grams .per square meter of the evaporation product of a water solution of diazotized para-aminodiphenylamine condensed with formaldehyde and converted to the double salt of zinc chloride, stabilized with in the range of 3.5 to 7.5 pants by weight of phosphoric acid per part by Weight of said diazo compound.

References Cited UNITED STATES PATENTS 1,983,005 12/1934 Sakurai 9691 Zahn 96-91X May 260-191 Frost et a1. 9633 XR Neugebauer 96-33 XR Neugeb-auer et a1 9633 Brinnick et a1. .96-75 Seven et a1. 3633 XR FOREIGN PATENTS Great Britain.

OTHER REFERENCES 15 Saunders, K..H., The Arom-atic Diazo Compounds,"

2nd ed., Arnold and Co., London, 1949, pp. 76-80.

J. TRAVIS BROWN, Acting Primary Examiner.

NORMAN G. TO-RCHIN, Examiner. R. L. STONE, Assistant Examiner. 

1. A PRESENSITIZED PRINTING PLATE COMPRISING A BASE MATERIAL HAVING THEREON A COATING COMPRISING (1) THE DOUBLE METAL SALT OF THE CONDENSATION PRODUCT OF A DIAZOTIZED PARA-AMINO-DIPHENYLAMINE AND FORMALDEHYDE AND (2) PHOSPHORIC ACID. 